Microstock photography does not require that you have the best, highest megapixel, or newest camera, but there are some things you should consider in your camera.
First: film or digital? A film camera will work, but using a film camera means you need to buy film, process film, and scan the negatives/slides into a digital format, which means you also need a decent scanner. These extra steps make it much more time consuming and costly to be a micostock photographer. This might not seem like a big deal, but if you are trying to build a portfolio of say a thousand pictures or more, costs will add up. If you are just getting started, you will probably save money by going digital. Digital cameras have dropped in price a great deal in the past few years, and you get a lot camera for a a few hundred dollars. You might also consider buying a used digital camera. With new cameras coming out so rapidly, a quality, not very old camera can be quite affordable. If you do go the used route, be very careful. Make sure the camera has been well cared for and has not been used to the point where it is ready to fail. With digital SLR (DSLR) cameras, you need to find out how many actuations (times the shutter has been fired). Many consumer DSLR shutters are rated to about 50,000, so if you are buying a camera with 25,000 actuations, you can consider half the life of the camera gone.
If you have some good shots taken with a film camera, you might consider scanning and submitting them. For example, you are a professional and have thousands of great images in your portfolio. Base your decision on the quality of your photos and your access to scanning equipment (personally owned or a service). When scanning your phots or slides, make sure you scan at a high enough resolution to give you a quality digital image to submit. For example, if you are scanning a slide, scanning at 2400 dpi is not overkill.
When you submit pictures, microstock sites have minimum requirements for the number of pixels in the pictures you submit. This applies to digital and film camera users. The requirements differ, depending on the site. Some sites require that the picture be no smaller than 600 pixels on the small dimension. Others require that your pictures be at least 3 or 4 megapixels. As you choose a camera, consider the requirements of the sites with which you want to work. Also consider that in many situations, you will want to crop your pictures. Make sure you have a camera with enough megapixels so you can crop and still meet the minimum requirements. For example, if your camera is capable of 4 megapixels and one of the sites with which you wnat to work requires 4 megapixels, you will not be able to crop your pictures at all and still meet the minimum size requirement.
Given two pictures that are equal in all aspects but size, the larger (inegapixels) the picture you can supply, the better. Many microstock sites offer the same photo in different sizes. The larger the picture purchased, the more it coasts and the more the photographer earns. If you can supply a bigger picture, the site will be able to make the picture available in the larger sizes. The more pixels, the more information there is in the picture, which means it can be blown-up larger without looking pixilated or grainy. When designers look for pictures, if they have a need for a large picture, they will be looking for a picture with a lot of pixels. If your camera is on the marginal end, you might want to save up and get a camera with more pixels.
How many pixels is enough? This is hard to say, every year, the cameras have more and more pixels. The more pixels that are placed within the same size sensot (pixel density), the more noise in the picture. Noise reduces the quality of the picture. To some degree, the image processing capabilities of newer cameras or software can help reduce the noise, but this can only go so far. At this point, you need a camera with a bigger sensor. Keep in mind when buying a camera that more pixels does not always mean your camera can take a better picture. For example, a high quality digital SLR with a quality lens and 8 megapixels can in most cases take a superior picture to a point-and-shoot camera with 10 megapixels and an OK lens. For one thing, the size of the sensor in the DSLR is probably significantly bigger than the one in the point-and-shoot, and the DSLR lens is probably superior to the point-and-shoot lens. Maybe it will suffice to say that you get what you pay for.
If you do not have a camera already or are considering upgrading, Consider getting a camera that can save a file in RAW format. The RAW format saves the picture information as the sensor sees it, without any processing. This allows you to adjust levels, sharpen, change white balance, and many other settings on the original data. A JPEG file already has a certain level of precessing done to it that can be difficult to remove if needed. Most DSLRs support this format and higher end point-and-shoot cameras support this also. However, this format requires a bit more processing on your part to get your pictures right, but it is far superior if you are considering being a professional or semi-professional. If you are not in one of these categories, the convenience of shooting JPEG files is hard to beat.